1. Technical Field
The present invention relates to remote sensing and more particularly to phase resolved shearography for remote sensing.
2. Background Information
The prior art discloses various means for detecting buried objects, for instance, non-phase resolved (NPR) shearography and laser doppler vibrometry. Non-phase resolved shearography is unable to resolve the phase of an acousto-seismic signal and requires the use of high power laser sources. This limitation means that (a) inversion methods cannot be applied to the output to identify scattering sources or targets of interest in the acousto-seismic signal, and (b) this type of system is impractical for aircraft and other fast moving platforms that require large area coverage rates.
There is a need, however, to remotely measure the full phase and amplitude information of small scale acousto-seismic vibrations in order to detect the presence of buried objects (e.g. mines, tunnels etc.) or for other purposes. This remote sensing information may need to be collected with a large area coverage rate and at a safe standoff distance.
In shearography, a target surface, part or area being observed is illuminated by an expanding laser beam, and two time sequential images are captured of the target surface, part or area with an image-shearing camera. The first image is taken of the surface, and the second image is taken of the same surface a short time thereafter during deformation or loading of the surface. The two images taken are processed together to produce a third image (a shearogram) showing a fringe pattern that depicts the gradient of the displacement of the surface due to some loading of the surface between the first and second images.
More particularly, shearography is an optical measuring technique using coherent light for the interferometric observation of the surfaces typically under non-destructive thermal or mechanical loading to distinguish between structural information and anomalies of the surfaces or parts due to loading such as thermal or mechanical loading. The two images are each laterally displaced images taken of the surface of the part being observed and the two images are coherently superposed. The lateral displacement is called the shear of the images. The superposition of the two images is called a shearogram, which is an interferogram of an object wave with the sheared surface wave as a reference wave.
The absolute difference of two shearograms recorded at different physical loading conditions of the target surface, part or area is an interference fringe pattern which is directly correlated to the difference in the deformation state of the target area between taking the two images thereof. In contrast to holographic interferometry, the fringe pattern indicates the slope of deformation rather than the deformation itself. Defects inside the target part will affect the local surface deformation induced by the loading and result in a disturbance of the loading fringes that are detected.
The resultant difference images always exhibit a very noisy structure. This is due to what are called speckles. Speckles are statistical interference patterns which occur after reflection of a coherent wave off a rough surface, giving the image a grainy structure. Regarding shearography, the speckles are the carrier of information, coding the wave field and surface state information respectively and giving rise to interference fringe patterns. However, the grainy nature of the speckles is conserved and significantly decreases contrast and signal to noise ratio of the difference images.
The difference images typically exhibit strong noise and low contrast that require further image processing. This further image processing can be either image improvement or image evaluation. The goal is to remove speckle noise and to increase fringe contrast in order to improve the visibility of the fringes.
U.S. Pat. No. 8,717,577 (incorporated in full herein by reference) provides a method of collecting shearography data for a subject target from a moving platform, such as an aircraft, surface craft, handheld device or moving vehicle. In short, said patent provides a process in which, during movement of the moving platform, two onboard laser transmitters and an onboard optical receiver are operated in a manner that they appear to be stationary. To capture the pair of images (specklegrams) required for shearography, the first laser transmitter transmits a first laser pulse toward the ground while positioned at an angle with respect to the ground. The resulting first ground image is captured by the optical receiver. A very short time thereafter, a second laser transmitter transmits a second laser pulse toward the ground at the same angle. This occurs because the position of the second laser transmitter has been adjusted according to the speed of the moving platform so that during the time between the first and second laser pulse transmissions, the second laser transmitter has moved a distance by the motion of the aircraft or other moving platform so that the second laser transmitter is at the same position during the second laser transmission that the first laser transmitter was during the first laser transmission.
The present method addresses the above-noted need and provides improvements which may be used in conjunction with the method of the above-noted U.S. Pat. No. 8,717,577. The present system and method may also be used in a variety of other contexts, as noted further below.